In many digital data applications, the transmitted data is converted into symbols in accordance with a predetermined process and the resulting set of all possible symbols from such a conversion process forms a signal constellation. There are many different conversion processes and signal constellations. For a symbol transmission rate, the data-carrying capacity of the data-to-symbol conversion process is directly proportional to amount of data represented by one symbol. Moreover, the greater the amount of digital data represented by one symbol, the greater the number of possible symbols. In most communications applications, the maximum power which can be transmitted is constrained and, as a result, as the number of possible symbols increases the separation between symbols decreases. Without the use of error correction schemes, the ability to recover the data after transmission through a noisy channel generally decreases with a decrease in the separation between symbols.
Many communications systems utilize a single data-to-symbol conversion process after a communications connection is established. For example, in most modems, during the "handshaking" process prior to data transmission, a particular data-to-symbol conversion process is agreed upon between the communicating modems and this process is maintained for the duration of the communications connection. In other system applications, the data-to-symbol conversion process may be varied during the communications connection in response to a determination of the data recovery error rate. That is, if the data error rate increases, the data-to-symbol conversion process can be altered so as to increase the separation between symbols and, therefore, decrease the likelihood of data recovery errors. In such arrangements, the change from one data-to-symbol conversion process to another occurs aperiodically.
More recently, in U.S. Pat. No. 5,214,656, issued May 25, 1993, two data-to-symbol conversion processes are used. More specifically, certain bits, which are deemed more important than other bits, are converted into symbols lying in a first signal constellation using a first data-to-symbol conversion process. This process provides greater error protection than a second data-to-symbol conversion process which converts the other bits into symbols in a second signal constellation. The symbols generated by each process are then transmitted in a time-division-multiplexed fashion. While this technique works satisfactorily, there are systems applications, such as the Asymmetric Digital Subscriber Line (ADSL), which require equal error protection for all transmitted data and which impose maximum delay requirements on certain transmitted data. In light of the expected widespread use of systems such as ADSL, it would be highly desirable if the above-described shortcomings of the prior art could be eliminated.